Subject: Aluminum powder (Arno Hahma; Gerald L. Hurst; Tom Perigrin)
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From: [email-masked] (Arno Hahma)
Subject: Re: Manufacturing metal Powders
Message-ID: <http://groups.google.com/groups?as_umsgid=Cv5CJy.6yC%40utu.fi>
Date: Fri, 26 Aug 1994 14:35:58 GMT

In article <[email-masked]>,
Wesley Brzozowski <[email-masked]> wrote:

>this, until I reached an unsettling conclusion. Aluminum, in a fine spray into
>the air, is surrounded by even more oxygen than it has available when surround

You might be interested to know, how they do it industrially: with
pressurized air or air that contains some steam in it. Molten aluminum
is still very far from its ignition point - there is really no
danger of getting the cloud to ignite. If moist air is used, the oxide
coating on the Al-particles will be even thicker and the powder will
become somewhat porous (rough and uneven surface profile with a lot of
area).

Additionally, you are not able to produce that fine Al dust with a
homebrew apparatus anyway - you'd have hard time trying to ignite the
cloud even with a propane torch or open flame. The industrial devices
utilize a ultrasonic crystal ("speaker") at the nozzle to make the
spray finer. Molten metals and especially aluminium have an incredibly
high surface tension, that tends to prevent small droplets from
forming.

About the ignitability of Al dust, you might try it on a small scale.
Even with the finest powders, you will discover a propane-oxygen or
oxyacetylene flame or equivalent is needed to get the cloud going. Or a
sparkler or burning magnesium...

>atmosphere, or something even less reactive. Magnesium can burn even
>in nitrogen.

For magnesium, yes. Mg ignites easily and even at its melting point.
Under nitrogen, you have to heat up to 700..800 oC, before the nitride
formation starts. So, it is possible to atomize Mg under nitrogen, but
there will still be some nitride present in it. The best solution is to
use argon. This is actually the reason Mg is normally ground
mechanically. Atomizing it is more expensive, though atomized Mg does
exist as well.

ArNO
2

Newsgroups: rec.pyrotechnics
From: [email-masked] (Arno Hahma)
Subject: Re: Manufacturing metal Powders
Message-ID: <http://groups.google.com/groups?as_umsgid=Cv6yA6.I5s%40utu.fi>
Date: Sat, 27 Aug 1994 11:22:54 GMT

In article <[email-masked]>,
Gordon Couger <[email-masked]> wrote:

>to form glass like metals. If you sprayed molten drops against a spinning
>cold plate instead of forming spherical drops it should form flat plates
>with much greater surface area.

Actually, you have invented one of the methods also used in the
industry: spinning plate atomization. A stream of molten metal is let
flow against a spinning plate, which sprays the metal into small
droplets. This process does indeed yield more flake or strip shaped
powder, not quite as spherical like the nozzle atomization does.
It depends much on the process parameters and the metal, though.

> Gordon Couger

ArNO
2

From: [email-masked] (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Powdering Metals
Date: 21 Mar 1996 06:24:59 GMT

In article <[email-masked]>, Dana Flewitt
<"umflewit "@ cc.umanitoba.ca> says:

>> >Al is probably much too malleable to be powdered in a ball mill. However,
>> >would it be possible to ball-mill atomized (spherical) Al to squish the
>> >particles in to a more flake-like powder?
>>
>> Yes. That's how it is done in industry. Aluminum is first atomized
>> in the liquid state and the product is ball milled to yield the
>> flake material.
>
>To obtain a powdered metal it is first supercooled so it becomes very
>brittle, almost to the point where it seems to lose some of its physical
>metallic properties, such as malleability. It is then crushed into a fine
>powder using various grinding techniques.

As I said, Al is atomized and later ball milled. Less active metals
can often be obtained by reduction of the oxides in a tube furnace
with hydrogen or carbon monoxide gas. Smaller quantities can be made
in batch processes in small glass ampoules or tubes containing the
metal oxide and a reducing agent. The open end of the tube is drawn
down to a narrow opening to prevent air diffusing back in. The tubes
are flamed with a burner and when the reaction is finished, the end
of the tube is sealed with a flame. Metals powders pepared in this
manner are often pyrophoric and will glitter as they ignite and burn
if the tube tip is broken and the contents sprinkled out.

Andrew is right that many metals can be ball milled at low temps.
I once had to use steel barrels as improvised containers for liquid
nitrogen baths for a pilot plant that was behind schedule. While
at liquid N2 temperatures, the metal of the barrels would shatter
like glass on impact.

Jerry (Ico)

From: [email-masked] (Gerald L. Hurst)
Newsgroups: rec.pyrotechnics
Subject: Re: Expensive Pyro AL
Date: 6 Jan 1997 20:34:07 GMT

In article <[email-masked]>, Zeppenwolf <[email-masked]> says:

>Sorry, but I can't help but wonder:
>
>a) Aluminum is ridiculously available
>b) Hydrochloric acid is ridiculously available
>c) a and b are ridiculously reactive
>
> so why not boil up some soda cans, evaporate,
>and use the sediment? Would it be too finely
>grained, or not work for some weird reason?
>
> I guess there must be *some* reason this won't
>work, or the pricey Al mentioned would never sell...

There have ben several posts recently which have suggested using
either acidic or basic aqueous solutions to convert solid or
particulate aluminum to finer particle size. This approach is not
viable because small particles have both higher chemical activity
(reactivity) and a larger surface area-to-mass ratio as compared
with larger particles. The result is that, on average, the smaller
particles disappear at a faster rate than that at which they are
formed from larger particles.

Another problem arises from the inherent tendency of some particles
to react more rapidly than othere as a result of random impurities
or geometry.

One might have better success using a solid-solid reaction where
available reactant is used up locally or physically separated
from the aluminum before the latter can completely disappear.

One method which I have not seen mentioned here is vacuum
deposition. Industrially, large vacuum chambers are employed to
produce metallized film such as is used for the so-called "Mylar"
balloons. The metal coat on these balloons is of the order of
0.25-1.0 microns thick. Much of the aluminum vapor produced in
these units does not strike the plastic surface as it rolls by and
is deposited randomly in as a fine waste powder throughout the
vacuum chamber.

Vacuum sputtering and similar techniques require nothing more than
a bell jar, a 10exp-5 vacuum and a source of electricity. In the
commercial vacuum deposition apparatus, aluminum in a refractory
boat is is heated by an electric current rather than a sputtering
arc. The aluminum vaporizes off into the chamber where it either
produces the shiny coat on the plastic or becomes dust. I do not
know the dimensions of the particles but I presume they are quite
small.

One of you (preferably one with some credentials, perhaps an
inartistic chemist) might easily obtain a sample of the aluminum
dust from any major coater if you wrote them a nice letter.
King-Sealy (Thermos), 3-M and Schjeldahl are (or were) particularly
knowledgeable in this area and are likely sources of additional
information.

One last word. High vacuum is required for shiny films but not to
make dust. A modest vacuum or no vacuum might work quite well if
the residual gas were helium, argon or neon. It would be
interesting to note the effect of pressure on the dimensions of the
aluminum "steam" particles.

Jerry (Ico)

Newsgroups: rec.pyrotechnics
From: [email-masked] (Tom Perigrin)
Subject: Re: atomized & granular
Date: Mon, 8 Jul 1996 21:33:03 GMT

In article <[email-masked]>, [email-masked] wrote:

> Could someone please explain the difference between atomized and
> granular, or any other terms used to describe particle size? For
> example, I understand that "air float" is a very fine if not the
> finest particle size (400-800 mesh?). I have read several books on
> pyrotechnics, but none of them covered this subject very well. Any and
> all info is greatly appreciated.

A little excerpt taken from "Introductory Practical Pyrotechnics", By Tom
Perigrin. This excerpt (C) Copyright, 1996, Falcon Fireworks.

-----

Aluminum, Al

Aluminum is often used as a metallic fuel, because it is inexpensive,
stable, and it gives off a large amount of heat when it is burned.
However, because Al2O3 is a white refractory solid which is often glowing
white or yellow hot when it is produced, the color balance of anything
other than white or sparks is degraded by this so-called ".i.black body;"
radiation.

Aluminum comes in a bewildering range of forms, sizes, etc...
Aluminum can be bright, gray, dark, or black. This is partly dependent
on the size of the particle, and also upon the method of manufacture.
The more finely divided forms of aluminum can be an inhalation health
hazard. The use of a face mask is recommended when performing
operations that may cause finely divided aluminum to become airborne.

Methods of manufacture and microscopic shapes of aluminum powders:

¥ Spherical or atomized aluminum is made by allowing a jet of gaseous
aluminum vapor to expand and condense in a chamber filled with inert gas.
The aluminum forms small balls or spheroids. Spherical aluminum is
available in many sizes, from -400 to -20 mesh.

¥ Flake aluminum is made in a number of different ways. It can be made
by ball milling or stamping. Stamped aluminum is made by placing
aluminum in a hard mortar, and using a mortar to stamp it flatter and
flatter. An inert oil is added to prevent the aluminum from sticking to
itself as new surfaces are exposed, and to reduce oxidation. Milled
aluminum is made by performing the same process in a ball mill. The action
is less compressive. It is said that stamped aluminum is denser, and
gives a different reaction than the milled.

¥ Flake aluminum can also be made by the expensive process of gluing
aluminum foil to sheets of paper and hammering it flatter and flatter.
This is the same process by which gold leaf is made. The paper is then
removed by charring in an inert atmosphere. This gives the black
aluminums, which are very reactive. Some people claim that black aluminums
are so reactive because of the combination of small particle size and the
formation of more reactive aluminum carbide (Al4C3) on the surface instead
of Al2O3.

¥ Granular aluminum is made by abrasive processes, including sanding or
grinding aluminum. The particles are more or less even in all three
dimensions, and have sharp ragged edges.

¥ Another term occasionally encountered is "aluminum flitters". This term
is used for very large flake aluminum, ranging from 15 to 40 mesh. This
aluminum is used in a formulation called "flitter", which produces a tail
of brightly burning aluminum particles.

Pure aluminum oxidizes in air. Thus it either has an oxide
coating or a coating of some inert material. Inert materials include
oils, stearin, or low molecular weight plastics. It is probable that
even aluminums with an inert coating have some thin oxide layer beneath
the inert coating. These coatings do prevent the buildup of a thick
oxide layer and so keep it "bright". Most paint aluminums have this
coating to keep them bright. If one takes a bright aluminum and heats
it in a furnace at 600¡ C under an inert atmosphere (the melting point
is 650¡ C), the coating boils off and one obtains a less-bright
aluminum that quickly goes darker on exposure to air.

Flake and granular aluminum are the most reactive aluminums,
with flake slightly leading granular. This is due to the sharp edges
of the flake and the corners of the granular particles. These can
quickly rise to a high temperature in a brief burst of flame, while the
bulk of the particle remains much cooler. The spherical aluminum
doesn't present any "opportunities" for rapid heating, and so the
entire particle must be heated to the ignition temperature before the
reaction commences.

Aluminum can undergo side reactions in the presence of nitrates
and water, to give ammonia and aluminum oxide. If the reaction becomes
too rapid it can lead to spontaneous combustion. Caution must be
exercised when wetting mixtures that contain aluminum and nitrates. If
the odor of ammonia is detected immediate steps must be taken to
prevent damage from fire or explosion. Fortunately the reaction can be
prevented by the addition of 1% of boric acid.

The formation of ammonia through the damp reaction of aluminum and
nitrates has led to many accidents. If chlorate is present this can lead
to the formation of ammonium chlorate, which is a very dangerous substance.
In fact, mixtures containing aluminum, a nitrate and a chlorate are given
the name "Death mixes".

Aluminum powders can be obtained from paint stores, in crafts
stores where it is sold as "bronzing powders", and in impure forms in
radiator sealers, etc.. The best varieties for pyrotechnics are
obtained from pyrotechnical supply houses which carefully separate and
distinguish between flake, spheroidal, spherical, and other forms.

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